The present invention relates to a process for preparing crystalline phosphate compounds having layer structure, and more particularly to a process for preparing crystalline layered hydrogenphosphates in a short period of time in a high efficiency, and use of the hydrogenphosphates as a metal ion adsorbent.
Hitherto, crystalline hydrogenphosphate compounds having layer structure have been prepared, for instance, by (1) a reflux process [A. Clearfield and J. A. Stynes, J. Inorg. Nucl. Chem., 26, 117(1964)] or (2) a fluorination complex process [J. M. Troup and A. Clearfield, Inorganic Chemistry, Vol. 16, No. 12, 3311(1977)]. In the reflux process (1), the crystalline hydrogenphosphates are prepared by refluxing amorphous phosphate in concentrated phosphoric acid for 24 to 100 hours. The process (1) requires the use of a large excess of concentrated phosphoric acid and a long period of time for the reaction and, therefore, it is economically disadvantageous for the industrial production. The fluorination complex process (2) also has a problem in safety because harmful hydrogen fluoride is used.
Also, it is proposed in Japanese Patent Publication Kokai No. 62-226807 to prepare crystalline layered zirconium hydrogenphosphate by hydrothermal synthesis. The hydrothermal synthesis requires high temperature and high pressure and, therefore, the proposed process has the disadvantage that the apparatus material must be resistant to a strong acid such as phosphoric acid under high temperature and high pressure when the process is practiced on an industrial scale.
The crystalline hydrogenphosphates have characteristics as a solid acid and are reactive with various cations, basic materials, organic amines and the like.
It is reported that titanium hydrogenphosphate and zirconium hydrogenphosphate are useful as an adsorbent for adsorbing cesium ion (ANS Topical Meeting, Fuel Reprocessing and Waste Management Proceedings 1984, Vol. II, pages 163-176). Sodium nitrate waste liquid from reprocessing of used nuclear fuel arising from atomic power stations contains cesium ion as a high level radioactive waste and, therefore, upon the disposal thereof it is necessary to remove or fix cesium ion, for instance, by selectively adsorbing it. However, according to the inventors' research, titanium hydrogenphosphate and zirconium hydrogenphosphate are not suitable for use as an adsorbent for removing cesium ion from such a waste liquid. Titanium hydrogenphosphate has an ability for adsorbing cesium ion, but it simultaneously adsorbs sodium ion. Because of poor selectivity, it cannot selectively and efficiently adsorb only cesium ion from a radioactive material-containing waste liquid which contains a large quantity of sodium ion, e.g. sodium nitrate waste liquid in reprocessing of used nuclear fuel. Zirconium hydrogenphosphate has little ability of adsorbing cesium ion and accordingly it has no practicality.
It is an object of the present invention to provide a process for preparing crystalline layered hydrogenphosphates economically in a shortened period of time in a high efficiency.
A further object of the invention is to provide a process for preparing crystalline layered hydrogenphosphates which are useful as adsorbent or inorganic ion exchanger, or are applicable to intercalation.
Another object of the invention is to provide an adsorbent capable of selectively and efficiently adsorbing cesium ion from a radioactive material-containing waste liquid.
These and other objects of the present invention will become apparent from the description hereinafter.